This invention relates to a hand held power driven tool and a drive system therefor for tightening threaded fasteners. The tool comprises a portable housing with an electric motor therein connected to drive an output shaft to which any selective one in a set of fastener engaging members is releasably connectable into working position.
Tools of that type have been introduced in the automotive industry for performing continuous repetitive work on assembly lines and include an electronic drive unit of solid state inverter type for each tool adapted to transform a single phase AC power input into three phase AC current to the motor. The frequency and tension of the current are automatically and continuously varied and controlled by the drive unit closely according to the demand of the tightening process performed by the fastener engaging member of the tool. In hitherto used applications, due to the automatic character of the operation the tool and drive unit have been set to tighten just one size or type of fasteners at a time, which means that on for instance an assembly line for engines or car bodies, the operator has been able to tighten fasteners in just a few positions on each object. This means that the operator is restricted to an undesirably monotonous work.
It is an object of the invention to enable use of the tool and drive unit on a whole set of fasteners of different sizes by enabling the drive unit to automatically adapt the parameters of the served electric power to the needs of the actual fastener.
This object is attained by the invention defined in the following claims and hereinafter described with reference to embodiments thereof illustrated in the accompanying drawings.